Prior spread spectrum systems have been developed that allow testing of live wires where a direct sequence spread spectrum signal at noise levels is added to the existing signal, transmitted down the wire, and a reflected signal is delayed by time. The reflected signal correlated to determine the characteristics and fault location of the wires. These systems perform poorly when the chip time of the spread spectrum signal (Tc) is less than the round-trip time for signals to travel over the wire (Tw).
For many applications, the constraint that Tc<Tw is not an issue. For example, one of the most interesting and useful applications has been for testing aging aircraft wiring where wire lengths are normally 10's to hundreds of feet in length.
However, there are many applications, particularly in life-critical medical devices such as defibrillators, pace-makers, and artificial heart pumps where the wires are very short (inches to a few feet), and are the least reliable component of the system. A method of testing these short wires in situ, capable of providing early prediction of failure would be of great benefit.
Prior art configurations requiring that Tc<Tw cannot address these applications. In fact, prior art cannot find a fault in a wire nearer than one chip time, or approximately 1*Tc+Tsc, where Tsc is the sub-chip time.